The present invention relates to an ionizing radiation curable resin composition that forms a lens sheet such as a Fresnel lens sheet, a lens sheet whose lens portion has been formed using that composition, and a projection screen that is equipped with that lens sheet.
Up to now, a two-sheet structure of projection screen that has been prepared by combining a lenticular lens and a Fresnel lens with each other is general as the projection screen. In this structure, to bond the both lenses to each other, it is practiced to assemble a lenticular lens sheet, which has been warped beforehand, to a planar Fresnel lens sheet in such a way as to press the former lens sheet against the latter lens sheet as illustrated in FIG. 1. Accordingly, a pressure is produced at a portion of contact between the assembled both lens sheets. For example, as illustrated in FIG. 6, in the case of a screen in which the lens surface of the lenticular lens the ridge of that has been vertically formed has been superposed upon the lens surface of the circular Fresnel lens, at the left and right parts of the screen line contacts mainly occur while at the vertical end parts of the screen mainly point contacts occur. In this projection screen, on the side of the Fresnel lens sheet that opposes the lenticular lens sheet, the portions each of that is substantially triangular in cross section and each apex point of that sharpens are concentrically arrayed. In the surface of the lenticular lens sheet that opposes the Fresnel lens sheet, the cross-sectional configuration thereof is formed into a semi-circular-columnar configuration. Accordingly, the material (resin composition) constituting the lens surface of the Fresnel lens sheet having the sharpening forward ends at their portions of contact needs to have a predetermined value or more of mechanical property with respect to the crush. This is because in case the lens surface is easily crushed with the above-described pressure, when that lens sheet has been used as the projection screen, excellent images cannot be obtained.
With respect to the above-described problems, in the specification of Japanese Patent Application Laid-Open No. 10-10647, there is disclosed a lens sheet the configuration stability of that is excellent over a wide temperature range and the optical properties of that can be maintained as are by setting the elastic modulus of the active energy radiation curable resin used in the lens portion of the lens sheet to be in the range of from 80 to 20000 kg/cm2 within a temperature range of from xe2x88x9220 to +40xc2x0 C.
Also, in Japanese Patent Application No. 2000-036435, the dissipation factor tan xcex4 of the dynamic visco-elasticity after curing of the ionizing radiation curable resin constituting the lens is set to fall within a predetermined range considering a case where dynamic force has been applied to the lens sheet to thereby provide a resin composition for lens sheet that has no strain built therein, flexibility, and excellent restorability.
However, in the specification of the above-described Japanese Patent Application Laid-Open No. 10-10647, the elastic modulus that is defined in JIS K-7113 is adopted. This elastic modulus is the one that is obtained by determining the value of the tensile elastic modulus through the use of a flat film, and therefore cannot be said to be the one that faithfully reproduces in an environment where an actual ionizing radiation curable resin is placed (receives a compression force).
Also, the pressure that when the resin composition is actually used as the projection screen occurs between the both lenses has a great length of duration. Namely, part of the resin composition for lens has a restoration force that acts to press that pressure back by degrees. Accordingly, when designing and selecting the resin composition for lens, it is necessary to take a time factor into consideration that should be involved in the mechanical properties of the resin composition.
Thereupon, the present invention has an object to provide a resin composition for lens sheet, the lens sheet, and a projection screen using it, which, by defining the mechanical properties based on the consideration of the pressure and time factor an ionizing radiation curable resin composition actually receives, even when any pressure is applied to the surface of the lens sheet, can obtain excellent images without having the lens configuration crushed due to that pressure.
Hereinafter, the present invention will be explained. In an aspect of the present invention, the above-described problems are solved by an ionizing radiation curable resin composition, the ionizing radiation curable resin composition forming a lens portion of a lens sheet, wherein the compression modulus of elasticity is greater than 0 MPa and smaller than 840 MPa; and the creep deformation factor is greater than 0% and smaller than 57%. The xe2x80x9celastic modulusxe2x80x9d and xe2x80x9ccreep deformation factorxe2x80x9d referred to here each mean a value that has been obtained by measurement performed using a xe2x80x9csmall degree of hardness testerxe2x80x9d that will be described later.
Also, in a second aspect of the present invention, the above-described problems are solved by an ionizing radiation curable resin composition, the ionizing radiation curable resin composition forming a lens portion of a lens sheet, wherein the compression modulus of elasticity is greater than 840 MPa and smaller than 3500 MPa; and the creep deformation factor is greater than xe2x88x9210% and smaller than 20%.
Further, in a third aspect of the present invention, the above-described problems are solved by an ionizing radiation curable resin composition, the ionizing radiation curable resin composition forming a lens portion of a lens sheet, wherein when E (MPa) represents the compression modulus of elasticity and C (%) represents the creep deformation factor, the ionizing radiation curable resin composition has a compression modulus of elasticity and creep deformation factor that have the relationship of
C less than xe2x88x922xc3x9710xe2x88x922E+63 and C greater than xe2x88x922.6xc3x9710xe2x88x923E+3 
According to each of these aspects of the present invention, it is possible to obtain an ionizing radiation curable resin composition that in case it is used for molding a lens surface, even when any pressure is applied to the surface of the lens sheet, can obtain excellent images without having the lens configuration crushed due to that pressure.
In a fourth aspect of the present invention, the Fresnel lens sheet may be constructed as a Fresnel lens sheet whose lens surface is formed of the ionizing radiation curable resin composition as described in one of the above-described aspects.
Further, in a fifth aspect of the present invention, the projection screen may be constructed as a projection screen that is equipped with the Fresnel lens sheet according to the above-described aspect.
The above-described functions and advantages of the present invention will be made clear from the embodiments that will be explained next.